Lead zirconate titanate containing lead is a typical piezoelectric material, and is used in a variety of piezoelectric devices such as an actuator, an oscillator, a sensor, and a filter. However, it has been pointed out that, when a piezoelectric device containing lead is once scrapped and exposed to acid rain, the lead content in the piezoelectric material may be transferred into the soil to adversely affect the ecosystem. Accordingly, in order to exclude lead from piezoelectric devices, research and development on lead-free piezoelectric materials are actively conducted.
NPL 1 reports that when a small amount of barium titanate is dissolved in sodium niobate, which is an antiferroelectric, sodium niobate becomes a ferroelectric. Further, NPL 1 discloses remanent polarization, a coercive field, a piezoelectric constant, an electromechanical coupling coefficient, and a mechanical quality factor obtained when a piezoelectric material containing barium titanate at a concentration of 5% to 20% is sintered at 1,200 to 1,280° C.
Further, the Curie temperature of the material in NPL 1 is higher than that (110 to 120° C.) of barium titanate, which is a typical lead-free piezoelectric material.
It is disclosed that the Curie temperature in a composition (Na0.9Ba0.1) (Nb0.9Ti0.1)O3 at which the largest piezoelectric constant d33=143 pC/N is obtained is 230° C.
On the other hand, there is a problem in that the mechanical quality factor in the composition (Na0.9Ba0.1) (Nb0.9Ti0.1)O3, at which the largest piezoelectric constant d33=143 pC/N is obtained in NPL 1, is 140, and this mechanical quality factor is lower than those of the other compositions.
In addition, PTL 1 discloses that the piezoelectric constant is improved by adding cobalt to piezoelectric ceramics as a solid solution of sodium niobate and barium titanate. On the other hand, the piezoelectric material of PTL 1 included a sample that is hardly polarized because of its low insulation property of 106Ω or less.